Optical signal equalizer for wavelength division multiplexed optical fiber systems

ABSTRACT

This invention disclosure describes the application of a polarization insensitive acoustically-tuned optical filter used in a multichannel WDM system to equalize variations in the power level of the WDM channels. The invention also describes a simple means for providing a low frequency control system which enables the equalizer to determine the signal levels of N optical carriers prior to equalizing the signals.

FIELD OF THE INVENTION

The present invention relates to equalization of wave-length-dependentoptical signals, and more particularly to equalization ofwavelength-dependent optical signals using a polarization-independentacoustically tuned optical filter.

BACKGROUND OF THE INVENTION

It is known that long optical fiber transmission links fortelecommunications can be built using cascaded chains of opticalamplifiers. Erbium doped optical fiber amplifiers are particularlywell-suited for implementing these long distance transmission systemsdue to their excellent performance characteristics and ease offabrication.

However, multiplexed optical-signals utilizing wave-length divisionmultiplexed (WDM) systems and erbium doped optical amplifiers exhibit avariation in signal gain that is a function of the individualwave-lengths. Moreover, utilizing cascaded optical amplifiers tocompensate for attenuation over the transmission link only exaggeratesthe variation in signal gain for the separate wavelengths. For example,a 10 channel WDM system with a 1 nm channel spacing could easily havinga gain variation over the 10 nm signal band of from 1 to 3 dB afteramplification. The total gain variation is increased by the product ofthe number of cascaded amplifiers, and thus will certainly be muchlarger. While a 1 to 3 dB gain variation may be acceptable for shortamplifiers chains, with 10 or more cascaded amplifiers the resulting 10to 10 dB gain variation is not likely to be acceptable.

Large variation in component signal levels of a multiplexed signal overthe wavelength spectrum complicates the design and performance ofoptical receivers and -detectors, and thus it is advantageous toequalize variations in signal level for any wavelength-dependentelements in the optical transmission path, particularlywavelength-dependent gain due to amplification.

OBJECTS OF THE INVENTION

Accordingly it is a primary object of this invention to obviate theabove noted and other disadvantages of the prior art.

It is a further object of this invention to control the optical signallevel of a optical signal composed of a plurality of differingwavelengths.

It is a yet further object of this invention to provide for uniformwavelength amplification of an optical signal composed of a plurality ofdiffering wavelengths.

It is a still further object of this invention to provide for automaticadjustment of an optical signal composed of multiple wavelengths.

SUMMARY OF THE INVENTION

The above and other objects and advantages are achieved in one aspect ofthe invention by including a polarization-independent acoustically tunedoptical filter (PIATOF) after a set of cascaded optical amplifiers toproduce a uniform signal level for each associated wavelength of theinput optical signal.

Multiple optical signals at differing wavelengths are combined by awavelength division multiplexor and passed through a series of opticalamplifiers. The output signal from the cascaded amplifiers is input to aPIATOF. A PIATOF is a two port output device, and the output of port oneof he PIATOF is tapped and the tapped signal is supplied to ademultiplexer to separate the input signal according to wavelength. Theresultant output signals of the, demultiplexer are input to a controlcircuitry. The control circuitry compares the output signal levels ofthe PIATOF for each wavelength and determines a proper RF power signalto be input at the control electrode of the PIATOF so that the signallevel for each wavelength of the output signal at port one of the PIATOFis uniform after the amplification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a fiber optic communication link includinga DFB laser and an optical amplifier.

FIG. 2 is an illustration of a fiber optic communication system inaccordance with the instant invention and including multiple lightsources emitting different wavelengths with a polarization independentacoustically-tuned optical filter for equalizing the output signal of anamplifier.

FIG. 3 illustrates another embodiment of the invention wherein aplurality of digital signals are combined with a low frequency referencecarrier, amplified, and input to a polarization independentacoustically-tuned optical filter which equalizes the signal level ofthe individual signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, wherein is depicted an optical fiber 110coupled to a lightwave source 120 such as a DFB laser with opticalintensity L at wavelength X. Optical Fiber 110 is coupled to an opticalamplifier 130, resulting in an optical gain and an output opticalintensity of L_(A). The gain, K=L_(A)/L, of optical amplifier 130 is afunction of the wavelength λ of the input lightwave. Thus if multiplewavelengths are combined in an input lightwave source and amplified, thegain for each wavelength after amplification will not be uniform, butrather dependent on the input wavelength.

Referring now to FIG. 2, wherein one embodiment of the instant inventionis depicted. N lightwave sources 210 transmit separate lightwave withwavelengths λ_(i). i=1, . . . ,n, which are input to a wavelengthdivision multiplexor 220 (WDM). The signals are combined in WDM 220 andtransmitted on fiber optic cable 110 to optical amplifier 130. Theoptical input signal to amplifier 130 is designated as L_(in), and asstated above is the combination of the individual lightwaves atwavelengths λ_(i). After amplification by amplifier 130, the resultantoptical signal L_(out), which is the sum of the individual amplifiedlightwaves at wavelengths λ_(i) does not exemplify a uniform opticalsignal at the individual wavelengths. The output signal L_(out) istapped at tap 230 and a portion of L_(out) is input into a 1×Ndemultiplexer 240 to isolate each separate lightwave at wavelengthλ_(i). The intensity of the optical signal from the output of thedemultiplexer is designated as L_(i) for each wavelength λ_(i). Theuntapped optical signal is input into a polarization independentacoustically-tuned optical filter 250 (PIATOF) which functions as amulti-channel splitter and equalizer. Polarization independentacoustically-tuned optical filters using wavelength divisionmultiplexing are described by D. A. Smith et al. in “Integrated-opticAcoustically Tunable Filters for WDm Networks” IEEE JSAC, Vol. 8 pgs.1151-1159, 1990, and D. A. Smith et al. in “Integrated-opticAcoustically Tunable Filters: Devices and Applications”, Optical FiberConference (OFC'91), San Diego, Feb. 18-22, 1991, p. 142, both of whichare incorporated by reference into this application.

PIATOF 250 has one input port, two output ports, and a control electrodefor determining the distribution of the input optical signal between thetwo ports. For the N WDM (λ₁, λ₂, . . . λ_(n)) wavelengths of L_(out)which are input to the PIATOF 250, each of the signals can be directedto either of the two output ports by applying an RF signal atfrequency_(i) to the control electrode of the PIATOF. The frequencyf_(i) is the corresponding frequency for wavelength λ_(i). Afterapplying RF power P_(i) at frequency f_(i), all the optical signal onchannel i at wavelength λ appears at port 2 of the PIATOF. Power P_(i)is determined emperically as it depends on construction of the PIATOF.Applying RF power X_(i)P_(i) at frequency f_(i), the optical signallevels corresponding to an initial lightwave intensity L_(i) appear atthe respective ports of the PIATOF.

OUTPIATOF₁=L_(i)cos²(X_(i)π/2)

OUTPIATOF₂=L_(i)sin²(X_(i)π/2)

Accordingly the optical signal level appearing at port 1 can beindependently controlled by applying a specified set of RF power levelsdetermined by a set of parameters (X₁. . . X_(n)) at frequencies f_(i)corresponding to the wavelengths λ_(i).

Continuing to refer to FIG. 2, PIATOF 250 is used as a means forequalizing the power levels of the optical signals resulting fromamplifier 130. After signal L_(out) is demultiplexed into the signalsL_(i), each signal L_(i) is input to a bank of n photodetectors 260 todetermine the signal levels of the L_(i) and is input to Control System280 to compare the respective levels. Control System 280 determines thecoefficients X_(i) for the respective wavelengths λ_(i) so as toequalize the output signal from PIATOF 250. RF power X_(i)P_(i) atfrequency f_(i) for each i=1, . . . , n is combined an input to thePIATOF on the control port of the device. The resultant output of thePIATOF displays a uniform signal for each wavelength λ_(i).

A further embodiment of the instant invention is shown in FIG. 3.Multiple transmitting DFB lasers 310 carrying conventional digital data(D₁, . . . , D_(N)) are modulated by low frequency control signals(ω_(i)=1 k to 10 k) with a small modulation depth from m=0.01 to 0.05.Each transmitting laser 310 is modulated by a separate control frequency(ω₁, . . . , ω_(N)). After combining the modulated input signals at WDMmultiplexor 320, the combined signal is passed through a series of fiberamplifier 330. After the cascaded amplifiers 330, a PIATOF 340 isinstalled in the transmission path. A 10 dB optical tap 345 is installedon output one of the PIATOF, and the tapped optical signal is providedto a single photodiode at photodetector 350. Photodetector 350 convertsthe tapped optical signal to an electrical signal, and demodulationcircuit 360 demultiplexes the signal into the signals C_(i) at the inputfrequencies ω_(i). Each signal C_(i) is input to Control System 370 tocompare the respective levels. Control System 370 determines thecoefficients X_(i) for the respective frequencies ω_(i) so as toequalize the output signal from the PIATOF 340. Applying RF powerX_(i)P_(i) at RF source 390 at frequency ω_(i) for each i=1, . . . ,n,the signals are combined and input to the PIATOF at the control port oneof the PIATOF is attenuated by the factor cos²(X_(i)π/2). By adjustingthe coefficient X_(i) for frequency ω_(i) for frequency ω_(i) of the RFpower at the control port, the output signal is equalized. Controlcircuitry 370 continuously monitors the modulated signal at thefrequencies (ω_(i), . . . . ,ω_(N)) providing dynamic equalization ofgain/loss due to elements in the network.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. Apparatus in a communication system forcontrolling the level of a multiplexed optical signal composed of aplurality of amplified optical signals Si S_(i) where i is a numberdenoting one of the plurality of optical signals at an individualfrequency fiwavelength λ_(i) comprising: tap means for tapping saidmultiplexed optical signal to produce a tapped multiplexed opticalsignal; means for converting said tapped multiplexed optical signal todemultiplexed electrical signals; control means having an input port forreceiving said demultiplexed electrical signals, including a calculationmeans for calculating coefficients Xi at frequency fi X_(i)corresponding to individual wavelengths λ_(i); filter means having aninput port for receiving said input multiplexed optical signal, a firstoutput port for transmitting a first output optical signal, andincluding a control port means for receiving said control signal;wherein the filter means responsive to the control signal attenuates thelevel of one or more of the individual optical signals S S_(i) at thefirst output port according to the relation cos²(Xiπ/2) cos²(X _(i)π/2).
 2. The apparatus of claim 1 wherein the filter means is apolarization independent acoustically tuned optical filter.
 3. Theapparatus of claim 1 wherein the control signal of the control portmeans in a multiplexed signal composed of one or more of the frequenciescorresponding to the wavelengths of the individual optical signals. 4.The apparatus of claim 1 wherein the control signal of the control portmeans is an electrical signal.
 5. The apparatus of claim 1 wherein thecontrol port means for receiving a control signal is an optical signal.6. The apparatus of claim 1 further comprising: a second output port fortransmitting a second output signal; wherein the filter meansdistributes the input optical signal between the first output port andthe second output port.
 7. The apparatus of claim 6 wherein the filtermeans distributes the input optical signal composed of a plurality ofoptical signals at separate wavelengths at the second output portproportional to sin²(Xiπ/2) sin²(X _(i) π/2).